Electrocatalytic and Photoelectrocatalytic Aspects of Proton Exchange Membrane Based Nanostructured Assemblies

Activity of electrocatalyst/support systems were analyzed in regards to proton exchange membrane fuel cells. Properties of a fuel cell membrane, Nafion, were investigated via spectroscopic and electrochemical measurements. Using the knowledge gained via the electrocatalyst/support and membrane studies, a photolytic reactor was created for the production of hydrogen. The PEM based catalysts systems discussed in this thesis provide new ways to efficiently disperse catalysts while allowing electrons, protons, liquids, and gases all access to the active sites.

Single walled carbon nanotubes were employed as a support for the reduction of platinum nanoparticles via sonolytic deposition and borohydride reduction. It was shown that borohydride reduction is a more efficient method than the sonolytic deposition method for platinum reduction in terms of fuel cell activity. Sheets of graphene oxide were also employed as a scaffold for borohydride reduced platinum nanoparticles. It was determined that hydrazine treatment of the graphene oxide increased the active catalytic surface area of the platinum nanoparticles, but may have a negative impact on the ability of the Nafion ionomer to efficiently transfer protons.

To broaden the methods in which graphene oxide could be reduced, photochemical and electrochemical reduction methods were investigated. It was discovered that graphene oxide could be photoreduced in the presence of TiO2 and methyl viologen. It was also found that the material could be reduced electrochemically. The ability for graphene to store electrons and then discharge the electrons was also investigated.

SiO2 was used as platinum supports for fuel cells. It was shown that varying the platinum to silica ratio was important in optimizing the electrochemically active surface area as well as the kinetics for the oxygen reduction reaction.

Methylene Blue was incorporated into a Nafion membrane in which it binds irreversibly to the polymeric backbone of the perfluorinated polymer. Membrane properties could be monitored via changes in the spectral response due to different protonation states of the methylene blue. The effective concentration of acid in the membrane was measured. Sodium ions were used to replace protons in the membrane and both transient as well as equilibrium effects within the membrane were studied. Spectral absorbance changes were also monitored with respect to changes in humidity.

Using an anodic TiO2 photocatalyst and a cathodic platinum electrode, a photolytic hydrogen reactor was built using the fuel cell concepts. A Nafion membrane transported protons while the external circuit enabled the transfer of electrons to travel from the photocatalyst anode to the cathode. At the cathode the protons and electrons recombined on the platinum anode to form hydrogen gas. The knowledge gained throughout the previous studies was essential in optimizing the efficiency of this device.